ExpressionEngine has a great Wiki Module built into all the packages except the Core version. Unfortunately at this present time (April 2009) the Wiki has two default theme that (in my opinion) are not up to scratch compared to the rest of EE. I’m going to use this article to link to themes for the Wiki Module that have been released to the public as I’ve struggled to find any decent ones.
Themes
Logical Blocks
Contented5
*I have not tested all these themes so they might be buggy, install at your own risk and expect to fix things.